In a hydraulic steering apparatus for construction vehicles which require extremely high power and flow rates of hydraulic fluid for steering purposes, linkage type and fully hydraulically operating type steering apparatuses comprising fully hydraulically actuated steering values have generally been used.
In a linkage type steering apparatus shown in FIG. 1, reference character "a" denotes a steering wheel, "b" a control valve, "c" a gear box, "d" a steering cylinder, "e" an arm, and "f" a pump. When the steering wheel "a" is turned by manipulation of the driver, the plunger in the control valve "b" is actuated through the linkage mechanism to allow fluid under pressure to flow into the steering cylinder "d" thereby actuating the piston in the steering cylinder "d" to turn the air "e". When the arm "e" is turned, a feed-back force is applied to the control valve "b" so that fluid under pressure is supplied through the control valve "b" into the steering cylinder "d" at a flow rate which corresponds with the turning speed of the steering wheel "a".
Whilst, the fully hydraulically operating type steering apparatus comprises a fully hydraulically actuated steering valve and a rotor. In this system, a control valve in the above-mentioned steering valve is actuated by the input from the steering wheel given by the manipulation thereof by the driven. As a result, the fluid under pressure delivered by the hydraulic pump is allowed to flow into the steering cylinder circuit to actuate the steering cylinder thereby turning the arm connected to the piston rod in the steering cylinder. At that time, the control valve receives a feed-back force which corresponds with the amount of turning of the rotor so that the fluid under pressure can be supplied into the steering cylinder at a flow rate corresponding with the turning speed of the steering wheel.
In the above-mentioned linkage type steering apparatus shown in FIG. 1, the displacement of the steering cylinder and subsequent component parts is fed back, and therefore a smooth operational feeling is obtained by the driver. However, since there are insensitive or dead zones such as backlash, etc. in the links and gears in the component parts up to the control valve "b", the play in the steering wheel "a" is considerably big, and there has been a problem relevant to steering controls, particularly when the vehicle is running at high speeds.
Whilst, the fully hydraulically operating type steering apparatus has characteristic features that since there is no linkage mechanism in the component parts up to the control valve the play of the steering wheel is small, and since the feed-back operation is made by the rotor the response of the hydraulic system is more excellent than that of the linkage type steering apparatus, and further because it is connected with the steering cylinder with hydraulic fluid pipings a large degree of freedom from the viewpoint of the layout is obtained, and therefore of late, steering apparatuses of fully hydraulically operating system have mostly been used instead of the linkage type ones.
However, in the fully hydraulically operating system, since the feed-back operation is made within the hydraulic system, and the entire systems in the hydraulic steering apparatus do not form a closed loop, there occurs non-synchronousness between the angle of turning of the steering wheel and the displacement of the piston in the cylinder. Stating in brief, the fully hydraulically operating system is disadvantageous in that, since it has no mechanical feed-back mechanism, leakage of fluid under pressure tends to cause non-synchronousness between the angle of turning of the steering wheel and the displacement of the piston in the cylinder.
In particular, in cases of vehicles such as fork lifts and shovel loaders wherein the steering wheel is provided with a knob, there has been a problem in that the position of the knob relative to the direction of advancement of the vehicle undergoes a divergence thus impairing the steerage thereof.